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# app/utils/calculator.rb
module StatelessCalculator
  def add(x, y)
    x + y
  end
  
  module_function :add
end

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class RssImporter
  def self.import(articles)
    articles.map do |article|
      RssContent.create { url: article.url, text: article.body }
    end
  end
end
 
# somewhere in your codebase...
imported = RssImporter.import a_bunch_of_rss_content

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# our first import
importer          = RssImporter.new
importer.articles = a_bunch_of_rss_content
importer.import
imported = importer.result

# second import, same instance of RssImporter
importer.articles = other_rss_content
importer.result # same value as "imported" on line 5

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class RssImporter
  attr_writer :articles
  attr_reader :result
   
  def import
    @result = articles.map do |article|
      RssContent.create { url: article.url, text: article.body }
    end
  end
end

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# app/utils/calculator.rb
class StatelessCalculator
  def self.add(x, y)
    x + y
  end

  # ...subtract, divide, multiply, etc.
end

# app/services/calculation_isolate.rb

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# app/util/prunable.rb
class Prunable
  def self.from_assets(assets)
    assets.select &method(:is_prunable?)
  end

  def self.is_prunable?(asset)
    asset.created_at < Date.today - 30
  end
end

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class S3FilePruner < JobProcessor
  def process_job
    Images.all.select do |image|
      image.created_at < Date.today - 30
    end.each do |image|
      S3Object.delete image.s3_object_name, image.s3_bucket_name
      image.destroy
    end
  end
end

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# Account = Struct.new(:id, :last_active)

describe 'pruning expired accounts' do
  let(:f) {
    # an account factory
    lambda do |id, last_active=Date.today|
      Account.new(id, last_active)
    end
  }
  

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# Account = Struct.new(:id, :last_active)

describe 'pruning expired accounts' do
  let(:acc1) { Account.new(1, Date.today) }
  let(:acc2) { Account.new(2, Date.today) }
  let(:acc3) { Account.new(3, Date.today) }
  
  describe 'when the first account is expired' do
    let(:acc1) { Account.new(1, Date.today - 45) }
    

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# app/controllers/calculation.rb
def create
  req = ActiveSupport::JSON.decode(request.body)
  
  calculator = StatefulCalculator.new
  
  # to_crunch => [[1, 2, 3], [9, 10, 11]]
  
  @sums = req["to_crunch"].map do |nums|
    NumberCruncher.new(calculator).crunch(nums)